1. Field of the Invention
The present invention relates to method to prepare a magnetic core-shell-shell nanoparticles having an iron cobalt alloy core, an intermediate silica shell and a manganese bismuth alloy surface layer on the silica shell having altered morphology obtained by nanoparticle synthesis within an applied magnetic field. This method offers further opportunity to prepare magnetic nanoparticles that may be tuned to have specific properties and thus provide a nanoparticle material suitable for preparation of a permanent magnet that is a rare-earth-element-free alternative to the standard neodymium iron borate permanent magnet material.
2. Discussion of the Background
The inventors are conducting a research program investigating both soft and hard magnetic materials obtained with nanoparticle materials obtained from wet chemical synthetic processes. Thus U.S. application Ser. No. 14/025,033, filed Sep. 12, 2013, discloses MnBi nanoparticles having a particle size of 5 to 200 nm as a source for hard magnetic materials. Additionally, U.S. Ser. No. 14/252,036, filed Apr. 14, 2014, discloses core-shell nanoparticles having an iron cobalt nanoparticle core of less than 200 nm with a silica shell and metal silicate interface as a source for soft magnetic materials. The disclosures of both Applications are incorporated herein by reference in their entireties. Further, in U.S. application Ser. No. 14/270,619, filed May 6, 2014, the inventors have disclosed core-shell-shell nanoparticles having a soft magnetic nanoparticle core of an iron cobalt alloy, a first shell of silica on the core and a further hard magnetic nano-coating of a manganese bismuth alloy. U.S. application Ser. No. 14/270,619 is incorporated herein by reference in its entirety.
Magnetic materials generally fall into two classes which are designated as magnetically hard substances which may be permanently magnetized or soft magnetic materials which may be reversed in magnetism at low applied fields. It is important in soft magnetic materials that energy loss, normally referenced as “core loss” is kept to a minimum whereas in hard magnetic materials it is preferred to resist changes in magnetization. High core losses are therefore characteristic of permanent magnetic materials and are undesirable in soft magnetic materials.
Many of today's advancing technologies require an efficient and strong hard magnet as a basic component of the device structure. Such devices range from cellular phones to high performance electric motors and significant effort is ongoing throughout the industry to find materials which not only meet current requirements, but also ever increasing demand for efficient, less expensive and easily produced hard magnet materials.
Conventionally, neodymium iron borate is generally recognized as one of the strongest, best performing hard magnet materials available. However, because this material is based on the rare earth element neodymium, it is expensive and often the available supply is not stable. Accordingly, there is a need for a material which performs equally or better than neodymium iron borate as a hard magnet but which is based on readily available and less expensive component materials.
Magnetic device parts are constructed from powders by compaction of the powders to a defined shape and then sintering the compact at temperatures of 200° C. or higher. Sintering the part following compaction, is necessary to achieve satisfactory mechanical properties in the part by providing particle to particle bonding and hence strength.
Technological advances in all aspects of the communications and power generation fields require ever increasing powerful magnetic powders having controllable or tunable magnetic properties which allow for production of tailored magnetic parts that are economical and readily obtainable.
Thus, an object of the present invention is to provide a method for preparing nanoparticle powders having magnetic properties that are tunable according to the controllable variable of the method.